Every major catastrophe seems to provide opportunities for new biological life forms. So it was with the Ordovician–Silurian Extinction. Up to that point, life had been bound to the seas, an environment that strongly limits both the penetration of sunlight and the diffusion of carbon dioxide (CO2). In the mild climate that followed the extinction, plants emerged from the aquatic habit at about 0.43 Ga and colonized land.
The ascent to land was a formidable undertaking. In their ancestral aquatic habitat, all cells were in close proximity to water and the nutrients dissolved in it. The restlessness of the surrounding fluid promoted mixing as well as diminished the boundary layer between an aquatic organism and its medium. Thus, absorption of water and nutrients by an aquatic organism did not quickly deplete their immediate surroundings.
On land, a desiccating atmosphere “sucked” water from organisms. Water and mineral nutrients were available primarily on the surface of or embedded in particles of soil. Soil was a difficult medium, one that was often hard to penetrate and that suffered from limited mixing and local depletion of water and nutrients. Plants in a terrestrial environment did, however, gain greater access to the critical resources of sunlight and CO2. In bodies of water, the amount of sunlight available for photosynthesis halves every 0.2 meters to 5 meters in depth, depending on the roughness and opacity of the surface and the density of photosynthetic organisms. Diffusion of CO2 is roughly 9000 times slower through still water than through air. On land, freed from these limitations, primary productivity literally skyrocketed. Vast jungles soon covered the continents.
These jungles, however, were only millimeters tall. In primitive plants such as liverworts, inefficient cell-to-cell mechanisms transport water and nutrient transport from the soil to aerial parts and carbohydrates from aerial parts to the light-deprived below-ground parts. Slow movement of materials thus restricted the height of such plants to a few millimeters. With the advent of a vascular system at about 0.38 Ga, plants were able to extend their reach. Vascular plants have specialized tissues that serve as efficient pipes through which water, nutrients, and carbohydrates rapidly flow for distances of meters and beyond. Once these transportation problems were addressed, competition for light became fierce, and plants grew in stature. Towering forests of vascular plants dominated the Carboniferous period (0.36 Ga to 0.29 Ga), so named because the organic matter from these forests eventually became the coal deposits that powered our Industrial Age.
Early terrestrial life. A forest of tree ferns, giant horsetails, scale trees, and associated creatures during the Carboniferous period.
The appearance of more complex plant life enhanced the amount of photosynthesis worldwide. Atmospheric concentrations of O2 rose from 15% to 30%, while those of CO2 declined from 0.5% to less than 0.05%. Lower concentrations of this important greenhouse gas were associated with falling global temperatures. By the end of the Carboniferous, from about 0.31Ga to 0.29 Ga, a new supercontinent Pangea (Greek; all earth) was developing from the merger of Gondwana and Euramerica, and glaciers covered all but tropical areas.
During the Permian period (0.29 Ga to 0.25 Ga), a greater percentage of Pangea straddled the equator. This ice-free landmass decreased the albedo (reflectance) of Earth and warmed the planet. The interior of the continent, without the moderating influence of the ocean, developed wide temperature fluctuations, highly seasonal rainfall patterns, and expansive deserts. The dry conditions favored gymnosperms—plants with seeds that enclose the embryos in a desiccation-resistant cover—over plants such as ferns that dispersespores that are vulnerable to drought.
The Great Dying
The Great Dying at 0.251 Ga, the most severe extinction event in the history of the planet, marks the end of an era (Paleozoic). Over 96% of marine and 70% of terrestrial species disappeared. Possible causes include an impact event in which a large meteor struck Earth , major volcanic eruptions , or methane release . Because no single cause for this devastation is evident, the convergence of several factors is suspected. In one scenario, massive volcanic eruptions released vast amounts of CO2 to the atmosphere and initiated global warming. 
This warming diminished the temperature gradient between the equator and the poles, which in turn disrupted circulation of the ocean surrounding the supercontinent Pangea. The stagnant water heated sufficiently to melt methane hydrate (ice that contains large amounts of CH4 within its crystal structure) in deep seabeds and produced a massive release of CH4 to the atmosphere. Global temperatures soared. Oxygen availability in the warm, stagnant ocean surrounding Pangea dropped, and sea life suffocated.
This is an excerpt from the book Global Climate Change: Convergence of Disciplines by Dr. Arnold J. Bloom and taken from UCVerse of the University of California.
©2010 Sinauer Associates and UC Regents